The invention concerns a multipole coil for influencing a particle beam, having at least two coils which enclose a concentric imaginary axis, wherein each coil has at least one winding made from a flexible circuit board and constructed by means of conducting paths disposed thereon, wherein the circuit board is rolled into at least one circuit board layer.
U.S. Pat. No. 3,702,450 discloses a multipole element of the above-mentioned kind. Towards this end, two coils, each having two halves, are disposed on either side of a beam and rotated through 90 degrees to deflect the beam in the x and y directions. This is especially useful for a scanning beam used to produce an image on a display. An example thereof is a television vacuum tube screen. The coils have substantially spiral shapes and extend from or end at a middle point. The coils are disposed on both sides of a side plate. They are then rolled-up, covered with an insulating layer, and introduced into a steel pipe. The object is to deflect a point-like particle beam. Coils having spiral shaped windings extending up to a middle point can be used for this purpose, since it is only necessary to subject the beam to sufficient electromagnetic forces to cause the deflection. These coils of prior art are however unsuitable for error correction in particle optics for the error-free transfer of images with highest resolution, since they do not produce pure multipole fields and cannot therefore satisfy the conditions for aberration correction. However, the main field of application of the invention is the influence of particle beams, i.e. ion or electron beams in particle optics with the goal of correcting aberrations. Since 1947, when O. Scherzer showed that the optical path of electron lenses can be corrected by non-rotationally symmetrical lenses (O. Scherzer: “Sphärische und chromatische Korrektur von Elektronen-Linsen” (spherical and chromatic correction of electron lenses), OPTIK, DE, JENA, 1947, pages 114-132, XP002090897, ISSN: 0863-0259) this type of correction has been generally used (David C. Joy “The aberration corrected SEM”—http://www.ccl.nist.gov/812/conference/2005_Talks/Joy.pdf or David B. Williams and C. Barry Carter “Transmission Electron Microscopy”, ISBN 0-306-45324-X, in particular page 93). Dipoles, quadrupoles, hexapoles and octupoles can be generated, in which the electromagnetic coils are disposed in such a fashion as to produce magnetic fields which are concentric to an imaginary axis, wherein the south and north poles are alternately disposed along the periphery. This enables influencing an electron or ion beam that passes through the coil arrangement in the area of the axis (optical axis) in order to correct aberrations.
The image correction suggested by O. Scherzer is based on correcting errors in particle optics caused by a deviation of the optical path from rotational symmetry, e.g. aperture aberrations and chromatic aberrations. The correction is based on the fact that multipoles influence a beam path in such a fashion that optical path distortions occur in two main cross sections under the condition that “beams traveling in two main sections which are perpendicular to each other and which leave the object center at the same angle with respect to the optical axis return to the center of the image at that same angle with respect to the optical axis” (Scherzer op. cit.). This means that distortion can be completely eliminated and an error-free image is therefore possible. (With respect to the influence of corrections, see Scherzer 1947 op. cit.). For the example of a point on the axis in the object plane, the fundamental principle is based on the fact that the beam path in an x-plane and a y-plane is such that two astigmatic intermediate images occur and a corrector is positioned for the beams in the x-plane where the basic path of the beam in the y-plane passes through the optical axis. The corrector for the beams in the y-plane is located where the fundamental path of the beams in the x-plane passes through the optical axis. In this manner, also corrections for off-axial beams can be performed, whereby other regions of the beam paths are also useable in the event that the beam paths are non-round at those locations. However, pure multipole fields must be present in the vicinity of the optical axis where the aberration correction is to take place, since the above mentioned condition is otherwise no longer fulfilled and an image distortion would remain which would render the image useless. The production of pure multipole fields is therefore an absolute requirement for aberration correction and the more precisely it is achieved, the better the image quality and the achievable image resolution.
The multipoles of prior art for aberration correction in particle optics correspond to the representation of FIG. 6.8(D) of David B. Williams and C. Barry Carter, page 93. These are webs that face towards the optical axis and are wrapped with conducting wires. Multipoles of this type require a large amount of space, comprise fields which are ineffective with respect to the area of the axis due to the large distance from the axis, and cannot be manufactured in an exactly reproducible fashion, since deviations cannot be prevented during winding of the wires both manually as well as mechanically.
In contrast thereto, U.S. Pat. No. 6,982,504B2, U.S. Pat. No. 4,271,370 and U.S. Pat. No. 3,736,543 disclose multipoles for the production of torques in motors, current measuring apparatus, or the like. Towards this end, the multipole elements have spiral windings, which extend up to the middle point of the spiral. One of the two current leads of the spiraling winding is disposed at the middle point or in the middle region. This configuration cannot produce pure multipole fields which, in accordance with the teachings of O. Scherzer (op. cit.), lead to a distortion free image after correction. This is also true for windings, which have windows, but which are covered by other displaced multipole windings, since the production of torques does not require pure multipole fields rather simply optimal torque production. Such electro-mechanic disclosures therefore provide no motivation to one of skill in the art of particle optics applications.
U.S. Pat. No. 3,587,019, U.S. Pat. No. 3,466,586 and U.S. Pat. No. 3,466,580 are concerned, as is U.S. Pat. No. 3,702,450, with the scanning of a beam. However, in contrast to the above-mentioned document, these documents do not have windings in coils rather produce the required fields for the scanning using conducting paths on circuit boards with meandering rectangular waves. These are superimposed on each other in a plurality of circuit board layers in such a fashion that multipole fields can be produced for the deflection of beams. These fields are also, however, much too inhomogeneous to produce distortion-free images in the sense taught by O. Scherzer.
EP 0,153,131 also discloses coil configurations for the production of multipole fields. The coil configurations, however, comprise pole-coils, which are wound about a cylinder in a slanted parallelogram manner. Due to the slanted orientation of the cylinder axis, these multipole fields are also not suitable for generating the distortion-free correction suggested by Scherzer in response to a deviation of the beam from rotational symmetry.
It is therefore the underlying purpose of the invention to introduce multipole coils of the above-mentioned kind, which are suitable for aberration correction in particle optics. The coils should require very reduced constructional space and produce effective fields in the region of the imaginary axis. Moreover, the multipole coils should be produced with high precision and reproducibility.